A laminated type cell is known as an example of a secondary battery. A laminated type cell has a construction in which positive-electrode sheets and negative-electrode sheets that are alternately and repeatedly stacked on each other with separators interposed therebetween. Each of the positive-electrode sheets and the negative-electrode sheets is configured an active material layer coated uniformly on a current collector. An example of a coating device for fabricating electrode sheets is disclosed in Patent Document 1.
An example of a relevant coating device and coating method are next described.
FIG. 8 and FIG. 9 are drawings showing an example of the configuration of a related coating device. FIG. 8 is a side view of the related coating device, and FIG. 9 is a top view of the principal parts of the device. In FIG. 8, moreover, a portion is shown in a transparent view for the purpose of explanation.
As shown in FIG. 8, the coating device includes: roller 210 which rotates in synchronization with the take-up speed of current collector 200 and through which current collector 200 is transported when current collector 200 that has been set in a pay-out portion (not shown in the figures) is being taken up toward a take-up portion (not shown in the figures); and die head 230 that dispenses coating slurry 220 that contains an active material upon current collector 200. Die head 230 has manifold 231 that stores coating slurry 220 that is supplied and slit 232 that dispenses coating slurry 220 that is stored in manifold 231 to current collector 200. Coating slurry 220 is supplied from a pump (not shown in the figures) to die head 230 by way of gate valve 250 and supply pipe 240.
Coating slurry 220 that is stored in manifold 231 is extruded to current collector 200 through slit 232 under the pressure of coating slurry 220 that is supplied from the pump (not shown). Coating active material layer 222 realized by coating slurry 220 is uniformly formed on current collector 200 by the take-up of current collector 200 toward the take-up portion (not shown) together with the rotation of roller 210.
As shown in FIG. 9, coating slurry 220 that flows into manifold 231 by way of supply pipe 240 is dispensed onto current collector 200 by way of slit 232 and coating active material layer 222 is formed on current collector 200. In the following explanation, the direction in which coating slurry 220 is dispensed from slit 232 is referred to as the coating direction. In addition, the direction perpendicular to the coating direction is referred to as the width direction of current collector 200, manifold 231, slit 232, and coating active material layer 222. In FIG. 9, the coating direction and width direction are indicated by arrows.
When an aqueous slurry is used as the coating slurry in the process of coating a negative-electrode, the problem arises that there are local sites, in which the coating weight of active material per unit area increases, in the width direction of the current collector.
FIG. 10 is a graph showing the coating weight of active material per unit area in the related coating method. The coating weight of active material per unit area was measured at 19 measurement points in the width direction of the current collector and the average value is taken as 100%. The horizontal axis of the graph shown in FIG. 10 shows the positions of the measurement points of the coating active material layer. The vertical axis of the graph shows in percentage the shift between the average value and the measured value at each measurement point.
Referring to FIG. 10, there clearly are sites at which the coating weight of active material per unit area is markedly greater than the average value, and further, sites at which the coating weight of active material per unit area is markedly smaller than the average value. The difference between the maximum value and minimum value of the coating weight of active material per unit area is approximately 3% of the average value of the coating weight of active material per unit area. The variation (standard deviation) is 0.70%.
An example of a method of making the thickness of the coating active material layer uniform is disclosed in Patent Document 2.
In the method disclosed in Patent Document 2, a flow adjustment plate is provided in a pocket that corresponds to the manifold of the die head. The flow adjustment plate is provided with a plurality of through-holes having differing aperture areas. The aperture area of through-holes is at a minimum in the vicinity of the coating material supply port of the pocket and the aperture area of through-holes gradually increases with increasing distance from the coating material supply port.